Power supply is an indispensable element on many information products. Its most important mission is to provide sufficient and stable voltage. Without this equipment cannot be started normally, and unpredictable voltage and current surges could take place to cause damage of the equipment.
The conventional power supply controls the duty cycle of a power driving unit by inputting a voltage through a pulse width modulation (PWM) circuit. By controlling the switch conduction cycle of the power driving unit, the peak-to-peak value of driving power can be regulated. The driving power is the voltage at two ends of the primary coil of the transformer in the power supply. The peak value and cycle of the driving power greatly affect output of the power supply. If the peak-to-peak value (Vpp) of the driving power generated by the power driving unit cannot be properly controlled or output error is excessive, the peripheral devices and semiconductor elements at the rear end could be damaged. To prevent such problems from happening, higher voltage-resistant semiconductor elements have to be used. As a result, the cost is higher. Hence controlling the peak-to-peak value (Vpp) of the driving power is an important research issue of power supply.
One of the conventional control methods that is widely adopted to control the PWM circuit is “voltage second tracking method”. It is based on a principle that during power conversion multiplication product of voltage and second is equal. Hence by tracking alterations of the driving power, and feeding back the driving power to the PWM circuit, the PWM circuit can regulate the duty cycle through the feedback voltage to stabilize the peak-to-peak value of the driving power. In practice, the voltage second tracking method uses two oscillators to independently control the cycle and voltage second. The oscillators consist of various elements such as resistors and capacitors. In practice, these elements have different characteristics and cannot be totally the same even made with same specifications. Every element has its error. As a result, the oscillators are not as accurate as desired and do not fully conform to their specifications. The error of the cycle and voltage second respectively controlled by the two oscillators are especially obvious. Hence there is always an error on the duty cycle output by the PWM circuit compared with that derived in the ideal condition. Without a mechanism to ensure the alteration range, the peak-to-peak value of the driving power output by the power driving unit is unstable. And the duty voltage with a constant peak value is not achievable. In the event that a transient surge or great load variation takes place, it could be mistakenly interpreted by the PWM circuit to generate a high voltage, and the rear end circuit or load could be damaged. To protect the safety of the circuit and load, more expensive high voltage-resistant semiconductor elements have to be used on the rear end circuit. It is not economic effective.